Nanya nih no 6,7 tolong banget nihhhhh

maaf klo kurang jelas sama klo salah .
cuma bisa jawab no 6

7. kelipatan 5 antara 300 - 500
305, 310, 315, ..., 495
a = 305
b = 5
Un = a + (n -1)b
495 = 305 + (n - 1)5
495 = 300 + 5n
5n = 195
n = 39

S39 = 39/2 (2(305) + 38(5))
= 39/2 (610 + 190)
= 39/2 (800)
= 15.600